The invention relates to a tool which may be used to gage the internal dimensions of polygonal tubes. More specifically, the invention relates to a mechanical gage assembly which may be used to determine the swelling due to neutron damage of tubes containing control rods in liquid-metal-cooled fast breeder nuclear reactors.
Nuclear reactors are usually controlled by rods containing material which affect the number of fissions taking place in the core. The rate with respect to time at which these fissions occur is directly proportional to the power produced as heat by the reactor. The rod material may absorb neutrons, thus decreasing the number of neutrons available to cause fissions; or the rod may contain nuclear fuel, thus providing more nuclei to be fissioned. In the former case, the rods must be moved into the core to decrease power, and in the latter, moved out of the core. Combinations of absorber and fuel in the same rod are also possible. In liquid-metal fast breeder reactors, these rods usually are contained within thimbles, which are polygonal tubes extending through the core; the tubes' purpose is to provide support and guidance for the control rods.
The intensity of the neutron radiation in the reactor core is measured by the neutron flux which is the total distance travelled in unit time by all the neutrons present in unit volume; the total distance is often referred to as the track length. In the course of travelling this track length, many neutrons pass through core structures, including the hexagonal thimbles which surround the control rods, prior to causing fissions or leaking out of the core; when passing through such core structures, collisions occur with the atoms of the structure. The main consequence of concern here is distortion of the crystal lattice of the metal structure, resulting in a noticeable increase in the volume of the structure after a period of time. In the case of a control rod thimble, this volumetric increase manifests itself in part as a decrease in the internal diameter of the thimble. Because the clearances between the rod itself and the surrounding thimble are small and precise, swelling due to neutron damage can bind control rods, preventing control rod movement. Controllability of the nuclear reactor may be seriously affected.
In the case of stainless steel materials, for instance, a 5% linear expansion resulting from a volumetric expansion of 15 to 50 vol. % can occur after the material has been exposed to a time-integrated neutron flux of approximately 10.sup.22 nvt. For hexagonal thimbles typically used to house control rods in liquid-metal fast breeder reactor cores, the dimensional change in the internal diameter of the thimble may be in the range of .+-. 76 .mu.m (.+-. 30 mils).
One method of measuring the thimble, which may be as much as 1.5 m (5 feet) long and 5.6 cm (2.2 inches) in diameter, is to use an inside caliper device, reading the calipers by means of a micrometer screw. However, such a device would require removal of the thimble from the reactor core since the caliper reading must be obtained while the device is in place on the tube. It is undesirable to remove the thimble from the core, since the core and thimble are submerged in a pool of radioactive liquid sodium at a temperature of approximately 500.degree. C. Also, handling the thimble outside the reactor would require shielded rooms and remote handling and measuring tools to avoid radioactive contamination from the thimble. Furthermore, shutdown time must be minimized as much as possible due to the economic consequences of the unavailability of a large commercial power-generating breeder reactor. The measurements could not be made by a device inside the core for the additional reason that liquid sodium is opaque and hence an indicating scale on the device could not be read.
Another possible means of determining the inside diameter of the hexagonal thimble would be to use a small-hole gage as described on pages 17 and 18 of Measurement Techniques in Mechanical Engineering, by R. J. Sweeney, (John Wiley and Sons, New York, 1953). The small-hole gage employs a single spring-loaded pin which slides in a larger body which supports the pin and spring. A handle is attached to the larger body. A central rod has threaded engagement with a hollow in the handle which communicates with the hollow in which the pin slides. The rod terminates in a knob at the top of the handle. By turning the knob, the rod may be brought into contact with the pin, which is then bound in position by the clamping effect. The gage is lowered into a hole with the pin aligned with the diameter or dimension of the hole to be measured. The rod is turned to release the pin, which springs out against the side of the hole. The pin is again clamped, and the gage is removed from the hole and the dimension determined by measuring from the face of the pin to the contact point on the body, opposite the pin face.
Another embodiment of the small-hole gage uses a split sphere, both halves of which are attached to the handle; the rod pulls a wedge into or pushes the wedge out of the space between the two halves, thus increasing or decreasing the sphere diameter. The spherical embodiment would give erroneous readings for distorted polygonal tubes since contact would not be made with all faces and would be difficult to withdraw once contact had been made with some of the inner walls of the thimble due to the thimble's long length and irregular distortion. The continuous sliding during withdrawal might wear the gage and destroy the accuracy of the readings. Additional error might result because the force pushing the halves against the tube is not necessarily constant, but depends upon the human operator. The pin embodiment might be used but would be more time consuming and subject to error. For a hexagonal thimble, three measurements and withdrawal and reinsertion after each measurement would be necessary in order to determine internal diameters of all the faces of the thimble at a single elevation. The present invention measures all faces simultaneously at several elevations. Furthermore, the combined action of the main and compensation springs built into the wedge structure of the present invention insures uniform operation of all plungers. It appears that dimensional tolerances would make it difficult to construct a multiple arrangement of the small-hole gage described by Sweeney.